1. Field of the Invention
The present invention relates to an inkjet recording head and an inkjet recording apparatus for carrying out the recording of characters and images with jetted droplets of ink.
2. Description of the Related Art
An inkjet recording head is used for printing characters and images onto a sheet of recording paper or the like by adhering droplets of inks onto the paper. The droplets of ink are selectively jetted from a plurality of nozzles while reciprocally moving the head in the main scanning direction as well as moving the recording paper in a sub-scanning direction orthogonal to the main scanning direction. FIGS. 19 to 21 are diagrams showing a structure of a general inkjet recording head according to a conventional technique. FIG. 19 is a exploded perspective diagram, FIG. 20 is a cross-sectional diagram of a portion near one pressure chamber, and FIG. 21 is a top plan perspective diagram of a main portion (piezoelectric actuators and pressure chambers). As shown in FIG. 19 and FIG. 20, the inkjet recording head is constructed of a nozzle plate 21, a supply-path plate 22, a pressure chamber plate 23, and a diaphragm 4, which are laminated in this order. Based on these plates and the diaphragm, there are formed plural ink paths extending from an ink pool 10 to nozzles 1 via supply paths 11 and pressure chambers 2, respectively.
Specifically, the plural nozzles 1 for jetting droplets of ink are formed in one line on the nozzle plate 21 with piercing through this plate. On the supply-path plate 22, there is formed each of the supply paths 11 that connects between a corresponding pressure chamber 2 and the ink pool 10, and each of through-holes 12 that connects between a corresponding one of the pressure chambers 2 and a corresponding one of the nozzles 1. Each of the supply paths 11 and through-holes 12 pierces through the supply-path plate 22. The pressure chamber plate 23 has the single ink pool 10 and the pressure chambers 2 corresponding to the respective nozzles 1, formed with piercing through this pressure chamber 23. Piezoelectric actuators 5 are connected to the diaphragm 4 corresponding to the respective pressure chambers 2 by a conductive adhesive. Electrode films are provided on both sides of each piezoelectric actuator 5, and an electrode film on the free surface side functions as an individual electrode 9. The diaphragm 4 made of a metal material also works as an electrode common to each piezoelectric actuator 5.
As shown in FIG. 20 and FIG. 21, each piezoelectric actuator 5 is shaped like a plate having a constant width, and this consists of a driving section 6 and an electrode pad section 7. The driving section 6 is positioned in an area corresponding to a corresponding pressure chamber 2, and the electrode pad section 7 is positioned in an area corresponding to a side wall 3 of the pressure chamber 2.
An electrical connection (not shown) from an external driving circuit to the individual electrodes 9 is made by the electrode pad section 7. When a potential difference is applied as a driving signal between both electrodes (the individual electrodes 9 and the diaphragm 4) of a piezoelectric actuator 5, the driving section 6 of the piezoelectric actuator 5 and an area of the diaphragm 4 corresponding to this driving section 6 are deformed. As a result, the ink in a corresponding pressure chamber 2 is compressed, and a droplet of ink is jetted from the corresponding nozzle 1. When the deformation is larger, it becomes possible to increase the jetted volume of ink. After the droplets of ink has been jetted, the ink is replenished from the ink pool 10 to the corresponding pressure chamber 2 via the corresponding supply path 11.
An electrical connection to the individual electrode 9 by the electrode pad section 7 provided in the piezoelectric actuator 5 has an advantage that no wiring is needed to provide an electrical connection to the driving section 6. With this arrangement, it is possible to prevent a constraint in the flexure deformation and the occurrence of variation in the deformation, which would be developed by the wiring at the time of driving. Further, the electrical connection made by the electrode pad section has another advantage as follows. The electrode pad section is not destroyed even when excessive pressure is applied to the electrode pad section in the electrical connection process, as the electrode pad section is located on the side wall of the pressure chamber and has high rigidity. In other words, it is possible to prevent the inkjet apparatus from being destroyed due to flexure caused by applying pressure.
As shown in FIGS. 19 to 21, conventionally the plane view of the pressure chamber has a rectangular shape. This has two reasons. One is that it has been desired to make the pitch between the nozzles as narrow as possible (that is, to make the short side of a rectangular shorter) so as to achieve high-resolution printing. The other reason is that it has been desired to make the flexure area of the diaphragm as large as possible (that is, to increase the long side thereof as much as possible) so as to secure the volume of an ink droplet required for the high-resolution printing. The piezoelectric actuator is shaped like a rectangular shape having a constant width to match the pressure chamber that has a rectangular shape.
As explained above, conventionally, a high-resolution inkjet recording head has been realized in a simple structure, by using a pressure chamber having a rectangular plan shape.
In recent years, a high speed has also been required for the inkjet recording head. In order to realize high-speed inkjet recording, it is effective to increase the number of nozzles. This is because when the number of nozzles is larger, it becomes possible to increase the number of ink droplets (dots of an image) that can be formed on the recording paper per unit time.
However, when only the number of nozzles is increased, the total size of the head becomes larger, and this brings about a problem of increase in the manufacturing cost of the head. Therefore, in the case of increasing the number of nozzles, it is necessary to take into account how to dispose as large number of nozzles as possible within a constant head area. In other words, how to increase the density of nozzles becomes a most important issue.
The pressure chamber occupies most of the area of each nozzle. Therefore, in order to realize the improvement in the nozzle density, it becomes essential to reduce the plane area of each pressure chamber. When the area of the plane of the pressure chamber is reduced, the flexure deformation of the driving section is lowered. As a result, the volume of a jetted ink droplet becomes smaller, resulting in reduced density of printed characters and images.
In other words, in order to realize the high-speed inkjet printing it is essential to increase the flexure deformation of the driving section even if the plane area of the pressure chamber is reduced, that is, to increase the driving efficiency per unit area.
It is an object of the present invention to provide an inkjet recording head allowing a high driving efficiency per unit area.
It is another object of the present invention to provide an inkjet recording head capable of preventing a variation in the driving efficiency even when a position of a piezoelectric actuator has deviated.
It is still another object of the present invention to provide an inkjet recording head that has high precision and high reliability, which can be manufactured at low cost in a simplified manufacturing process.
In order to solve the above problems, the inventors have carried out analysis and research focusing attention on the plane shape of a pressure chamber. On the condition that pressure chambers having rectangular plane shapes of the same plane areas but having different aspect ratios (the ratio of height to width), we measured an amount of flexure deformation when these pressure chambers provided with diaphragms and piezoelectric actuators have been vibrated. FIG. 4 shows a result of the flexure deformation measurements. FIG. 4 also shows states of the flexure deformation of the piezoelectric actuators. In this case, an aspect ratio may be an index that shows a degree of plane shape flatness of the pressure chamber. Specifically, the aspect ratio is defined as B/A for each of shapes shown in FIG. 3. When this numerical value is large, this means that the pressure chamber has a slender plane shape. For example, the aspect ratio of an equilateral triangle is 0.866, the aspect ratio of a regular square is 1, the aspect ratio of a regular hexagon is 0.866, and the aspect ratio of a true circle is 1. Conditions for the analysis are as follows. The plane area of a pressure chamber is 2.5xc3x9710xe2x88x927 m2. The thickness of a diaphragm is 10 xcexcm. The material of the diaphragm is stainless steel SUS304. The thickness of an piezoelectric actuator is 30 xcexcm. The material of the piezoelectric actuator is PZT. The shape of the piezoelectric actuator is the same as that of the pressure chamber (an electrode pad is not included). The driving voltage is 30 V.
It has been known from the result shown in FIG. 4 that an optimum aspect ratio of the pressure chamber is 1, in order to obtain a high driving efficiency per unit area. Based on this result, a more practical structure is assumed. An additional analysis has been carried out for the case where an electrode pad has been provided in the piezoelectric actuator. An electrode pad section of each shape has been provided on the short side of the plane shape of the pressure chamber.
FIG. 5 shows a result of this analysis. For the purpose of comparison, the result shown in FIG. 4 is also included in FIG. 5. It has become clear from FIG. 5 that the driving efficiency is lowered when the electrode pad is additionally provided. The amount of reduction depends on the aspect ratio. Particularly, such reduction is extreme when the pressure chamber has a structure having the aspect ratio close to 1. In other words, when the electrode pad is additionally provided, there arises a specific problem due to the shape having the aspect ratio close to 1. It has become clear that the effect of the improvement in the driving efficiency is small when only the aspect ratio is set close to 1. Accordingly, it is necessary to make further device in order to obtain more effect.
Before considering means for achieving such, a cause of the reduction in the efficiency due to the addition of the electrode pad has been studied. The states of flexure deformation are compared by observation between the case of the presence of an electrode pad with the case of the absence of an electrode pad in FIG. 4 and FIG. 5. As a result, it is understood that the deformation is lost at the connection portion between the electrode pad section and the driving section. From this, it is considered that the efficiency is lowered as the electrode pad section constrains free deformation of the driving section. Particularly, in the structure having the aspect ratio near 1, the cross-sectional area of the connection portion between the driving section and the electrode pad section is large. Therefore, it is considered that this large cross-sectional area constrains the deformation, and extremely lowers the efficiency.
From the result of the above research, it can be understood as follows. In order to improve the driving efficiency per unit area, it is important to use pressure chambers having a plane shape with the aspect ratio close to 1, and to realize a structure having small constraint on the electrode pad section.
In order to achieve the above objects, according to one aspect of the present invention, there is provided an inkjet recording head in which each actuator is comprised of: a driving section that is disposed in an area corresponding to a pressure chamber, and that is deformed in flexure together with a diaphragm when a driving signal is applied; an electrode pad section that is disposed in an area corresponding to a side wall of the pressure chamber, and that carries out an electrical connection with a driving signal source; and a bridge section that connects the driving section and the electrode pad section. In this inkjet recording head, the pressure chamber has a plane shape having an aspect ratio approximately equal to 1. In the bridge section, the width of a connection area to the driving section is smaller than the width of a connection side of the driving section. According to this aspect, it is possible to lower the constraint of the electrode pad section when the driving section deforms in flexure, and it is possible to prevent a reduction in the flexure deformation. Therefore, it is possible to realize the inkjet recording head having high driving efficiency.
In the above aspect of the invention, it is preferable that the width of the connection area to the driving section is reduced to a size equal to or less than one half of the width of the connection side of the driving section. Based on a very small connection area between the driving section and the electrode pad section, it is possible to substantially cancel the constraint of the electrode pad section when the driving section deforms in flexure, and prevent a reduction in the flexure deformation. Therefore, it is possible to realize the inkjet recording head having high driving efficiency.
Further, according to another aspect of the present invention, there is provided an inkjet recording head in which one or a plurality of bridge sections are connected to a driving section at a portion corresponding to the vicinity of a portion having small flexure deformation of a diaphragm. Further, one or a plurality of bridge sections are connected to a driving section at a position with a distance from the center of a connection area side of the driving section. Further, one or a plurality of bridge sections are connected to a driving section at a portion corresponding to the vicinity of the top of the pressure chamber. These portions are at the positions where basically the diaphragm is little deformed. Therefore, even when the electrode pad section is connected to the driving section by providing bridges in the vicinity, there is substantially no influence that the electrode pad section constrains the flexure deformation of the driving section. As a result, it is possible to obtain large deformation. With this structure, the bending deformation of the bridge itself is small. Therefore, it is possible to prevent the occurrence of cracks in the bridge section and breaking due to fatigue.
It is possible to structure the edge portion of the area of the connection with the driving section of the bridge section in a curve. With this arrangement, it is possible to relax the stress concentration in the vicinity of the connection portion of the bridge section at the manufacturing time or at the time of driving flexure deformation. As a result, it is possible to prevent the destruction of the actuator. It is also possible to form the edge of the connection portion between the bridge section and the electrode pad section in a curve.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head in which there is the following relationship: Wpxe2x89xa6Wcxe2x88x922xcex4, or Wc+2xcex4xe2x89xa6Wp, where xcex4 represents a positional deviation between a center position of a pressure chamber and a center position of a driving section, Wp represents a width of a plane shape of the driving section, and Wc represents a width of a plane shape of the pressure chamber. A Wc value corresponds to a value of A for each plane shape as shown in FIG. 3, for example. In general, the flexure deformation of the driving section is largely subjected to the influence of supporting conditions of the external periphery of the driving section. For example, it is possible to obtain large flexure deformation based on a rotation-free support, in a structure where the actuator is not applied to the external wall of the pressure chamber (the actuator is smaller than the pressure chamber). On the other hand, flexure deformation is small based on a fixed support, in a structure where the actuator is applied to the external wall of the pressure chamber (the actuator is larger than the pressure chamber). Assume that there exist the following two states. One state is that the driving section is applied to the external wall of the pressure chamber. The other state is that the driving section is not applied to the external wall of the pressure chamber, due to the positional deviation of the piezoelectric actuator under the disturbance in the manufacturing process. In this case, there is a large difference in flexure deformation between the two cases. In other words, the variation becomes large. According to this aspect of the invention, when Wpxe2x89xa6Wcxe2x88x922xcex4 is satisfied, the driving section is not applied to the external wall of the pressure chamber any time even when there has been a positional deviation in either direction. Therefore, it is always possible to keep the rotation-free supporting condition. In the mean time, when Wc+2xcex4xe2x89xa6Wp is satisfied, the external periphery of the driving section is always kept applied to the external wall of the pressure chamber even when a positional deviation has occurred. Therefore, it is always possible to keep the fixing supporting condition. As a result, when any one of these conditions is satisfied, the variation in the flexure deformation attributable to the positional deviation becomes small, and it is possible provide a high-precision inkjet recording head.
In the above aspect of the invention, it is more preferable that Wp is in the following range: (Wcxe2x88x922xcex4)xc3x970.9xe2x89xa6Wpxe2x89xa6Wcxe2x88x922xcex4. In general, under the same rotation-free supporting condition, the flexure deformation becomes small when Wc is smaller than Wp, as the flexure deformation area is small, and the flexure deformation becomes small when Wp is closer to Wc, as the supporting condition becomes close to the fixed support. In other words, Wp has an optimum value relative to Wc. According to this aspect of the present invention, as Wp can be set to an optimum value, it is possible to maximize the flexure deformation. At the same time, it is possible to minimize the variation in the flexure deformation relative to the positional deviation of the piezoelectric actuator. As a result, it is possible provide a high-precision inkjet recording head.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head in which a plurality of nozzles are disposed two dimensionally. Further, a plurality of nozzles arrayed in one row with a constant interval between the nozzles, are disposed in a plurality of rows. Based on only a one-dimensional layout of nozzles, it is not possible to make the nozzle layout pitch smaller than the width of the pressure chamber. Therefore, it is not possible to realize a high-resolution inkjet recording head. However, according to this aspect of the present invention, it is possible to make the nozzle layout pitch smaller than the width of the pressure chamber. Therefore, it is possible to realize a high-resolution inkjet recording head.
According to the two-dimensional layout, the nozzles disposed at constant intervals in a row are arranged in N rows in a direction approximately orthogonal to the scanning direction of the inkjet recording head, for example. The layout of the nozzles in each row is sequentially differentiated by a constant distance of 1/N in a column direction. Alternatively, the nozzles in each row may be arrayed in equal distances so that the nozzles are disposed at crossing positions of lattices of parallelograms. According to the layout of this aspect of the invention, the nozzles are projected in a direction orthogonal to the scanning direction of the recording head (refer to FIG. 14). Based on this layout, as compared with the case where the nozzles are disposed one dimensionally, it is possible to narrow the pitches of the nozzles (the nozzle layout pitches) to 1/N of these pitches. In other words, it is possible to provide a high-resolution inkjet recording head.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head of the above aspect in which a wiring substrate including a signal line is disposed to cover actuators that are disposed two dimensionally in a matrix. Further, the electrode pad section and the wiring substrate are electrically connected to each other via a bump. According to this aspect of the invention, a signal line to each piezoelectric actuator exists at the outside of the plane of each piezoelectric actuator. Therefore, it is possible to lay out the signal lines in high density without the need for each signal line space that has conventionally been provided between the actuators.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head in which a bump is constructed of a conductive core material and a connection material with which the periphery of the core material is coated. According to this aspect of the invention, a clearance is formed between the wiring substrate and the piezoelectric actuator driving section. Therefore, the wiring substrate does not give influence to the flexure distortion of the driving section. Further, according to this aspect of the invention, the heated driving section due to the driving of the piezoelectric actuator is cooled with the air that flows through the clearance.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head of the above aspect in which the core material is formed in a semi-spherical shape. According to this aspect of the invention, it is possible to ensure electrical and mechanical contact with the electrode pad section. Further, according to this aspect of the invention, it is possible to prevent a destruction of the electrode pad section in the process of forming a contact with the electrode pad section.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head of the above aspect in which a wiring substrate is constructed to include at least a resin base material. According to this aspect of the invention, it is possible to prevent a destruction of the bump, even when the inkjet recording head has been expanded or warped due to a temperature change, as the wiring substrate of the resin base material has low rigidity.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head of the above aspect in which the actuator is a piezoelectric actuator having a driving section made of a piezoelectric element. For manufacturing the piezoelectric actuator, a sandblasting method (to be described later) is applied. Based on this method, it is possible to process the piezoelectric actuator easily and precisely in a short time, even if the piezoelectric actuator has a complex shape having a plurality of bridge sections. As a result, it is possible to realize high-density ink jetting at low cost.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head in which a dummy pattern is disposed to cover the external periphery of a piezoelectric actuator area where a plurality of piezoelectric actuators are laid out, and/or is disposed between the piezoelectric actuators. In general, the sandblasting method has a problem of size precision in the processing called sand etching. There is a film mask portion that is left in an area (each actuator, in the present invention) where grinding with sandblasting is not carried out. Blast grinding particles exist below the mask in the vicinity of the edge of this film mask portion, and these grinding particles are also ground. As a result, there occurs a variation in the finish processing size. This sand etching depends on presence or absence of an adjacent item to be processed. More specifically, the sand etching depends on a distance between the adjacent items to be processed. According to this aspect of the present invention, a dummy pattern exists on the external periphery of the piezoelectric actuator area. Therefore, there is little difference in the sand etching between the external periphery and the inside of the piezoelectric actuator area. Consequently, it is possible to obtain a uniform size, and it becomes possible to realize high precision. Further, according to this aspect of the invention, a dummy pattern also exits on the surrounding of each piezoelectric actuator. Therefore, there is little difference in the sand etching among all the piezoelectric actuators. As a result, it becomes possible to realize high precision.
Further, according to still another aspect of the present invention, there is provided an inkjet recording head in which a width of a groove that separates between a piezoelectric actuator and an adjacent dummy pattern (an isolation distance) is set substantially the same for all the grooves. According to this aspect of the invention, the sand etching becomes the same for all the piezoelectric actuators. Therefore, it is possible to obtain a uniform size. As a result, it becomes possible to realize high precision.
Further, according to still another aspect of the present invention, there is provided an inkjet recording apparatus that is mounted with any one of the inkjet recording heads according to the above aspects of the invention.